1. Field of the Invention
The present invention relates to a control circuit for conducting control to protect a recording head of an ink jet recording apparatus when the power for a power supply circuit thereof is interrupted.
2. Description of the Related Art
A power supply (power supply unit or power supply circuit) of an ink jet recording apparatus (printer) needs to supply at least two voltages, one for driving a recording head and the other for operating a driver circuit (control circuit). Said power supply is required to prevent, for example, disconnection in the driver circuit by dropping the voltage for driving the recording head before the voltage for the driver circuit drops if an input power supply (commercial power supply) is turned OFF during recording, for example.
The electronic components such as a CPU or an IC used in such a control circuit have a lowest voltage at which they are guaranteed to operate. At a voltage below this guarantee voltage, such CPU or IC will be reset. As a result, the motor or the recording head of the apparatus may malfunction in some cases.
To guard against this, as mentioned above, typically the control circuit is devised to prevent these components from malfunctioning upon voltage drop due to power-OFF or power interruption. One example of such a device may be a reset function, by which the CPU or IC is initialized when the voltage of the control circuit reaches a reset voltage. By this initialization, such device as a motor or recording head can be entered into a safe state.
The conventional printer power supply has a multiple-output configuration for obtaining a plurality of outputs on the side of the secondary winding of a converting transformer of a switching power supply, in which the output of these which is used for driving a recording head is in some cases provided with an output ON/OFF switch for using a control signal (ON/OFF signal) sent from the side of the apparatus to turn ON/OFF the supply of power in order to save on power dissipation in the standby state of the apparatus or to preserve safety in an event where a service person or a user has touched the apparatus.
To stabilize the voltage applied on the recording head, the power supply circuit of such a printer adds capacitance by use of an electrolytic capacitor to the apparatus and the vicinity of the recording head. Such a capacitor needs to be large in capacitance to accommodate an improvement in the performance of the printer.
For this purpose, such a configuration is implemented that a discharge circuit is provided so that if the input power supply is interrupted, immediately after the voltage on the driver circuit drops to reset the components, the output side of the switch may be discharged of the capacitance load, to drop the voltage on the recording head instantaneously in order to prevent disconnection etc. thereof.
The operating waveform of the output voltage in such a case is shown in FIG. 8. In FIG. 8, V1 indicates the recording head driving voltage and V2, the driver circuit voltage.
A suggestion for protecting the recording head further is disclosed in Japanese Patent Publication No. 2000-102248. The configuration of a power supply circuit according to said suggestion is shown in FIG. 10 and its operating waveform, in FIG. 9.
If, during the operation of an apparatus using this switching power supply device, supply of the input power is stopped by breaking of the input switch, for example, when the plug of the apparatus is pulled out accidentally, the second output V2 has roughly a constant current value (rated current value) no matter whether the apparatus is operating or not, so that the output voltage drops readily.
To guard against this, a detector circuit 18 is provided for detecting a drop in the primary-winding side rectified voltage (Vin) before the second output voltage V2 starts to drop. When the detector circuit 18 detects a drop in a voltage Vin to then output a detection signal, correspondingly said first ON/OFF switching circuit 19 is forcedly turned OFF and, at the same time, an output voltage discharge circuit 20 is turned ON. By this process, the voltage at the first output terminal can be dropped instantaneously no matter whether a load current is flowing or not.
As indicated by the waveform of FIG. 9, this circuit has such a characteristic that as the DC voltage Vin on the primary-winding side capacitor drops gradually upon interruption of the supply of power, with the switching operation continuing by the switching power supply, an output ON/OFF switch 3 is forcedly turned OFF before the output voltage drops below a rated value (moment t0), to release the load capacitance.
By this process, even before the value of the second output V2 starts to drop, at any value of the rated load current of each output the head driver power supply voltage V1 can be dropped to thereby lower the head driving voltage V1 down to a safe voltage Vsafe at a timing (Trst), at which the voltage V2 drops to a reset voltage.
As such, a circuit shown in FIG. 10 constitutes power supply means for preventing disconnection etc. of a circuit in the recording head by dropping the recording head driving voltage before the voltage on the driver circuit is reset.
Recently, however, the printer has been improved in performance and an electrolytic capacitor used therein has been increased in capacitance. To drop the head driving voltage rapidly against load of this large capacitance, it is necessary to discharge the head in a large current instantaneously. For this purpose, the switching elements and peripheral components for discharge must have a very large value of allowable power. Moreover, a discharge circuit and an input power supply-interruption detecting circuit must be added to the switching power supply circuit, to complicate the configuration of the power supply circuit and increase the number of necessary components, the area for mounting these, and the volume of the power supply circuit, thus increasing also the manufacturing costs.